I am new to reloading and I have a dumb question..

Looking through the Barnes Reloading Manual I noticed that for a 150 grain XBT bullet for both the 308 and 7mm, the 308 flies faster and flatter.For the same type of powder they suggest loading the7mm with less powder than the 308? Why is that?

With the improved BC of the 7mm, I would have expected a performance improvement over the 308 bullet.